Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Changes in threshold voltage of the memory cells, through programming of charge storage structures (e.g., floating gates or charge traps) or other physical phenomena (e.g., phase change or polarization), determine the data value of each cell. The cells are usually grouped into blocks. Each of the cells within a block can be electrically programmed, such as by charging the charge storage structure. The data in a cell of this type is determined by the presence or absence of the charge in the charge storage structure. The charge can be removed from the charge storage structure by an erase operation. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, digital media players, digital recorders, games, appliances, vehicles, wireless devices, cellular telephones, and removable memory modules, and the uses for flash memory continue to expand.
Flash memory typically utilizes one of two basic architectures known as NOR flash and NAND flash. The designation is derived from the logic used to read the devices. In NOR flash architecture, a logical column of memory cells is coupled in parallel with each memory cell coupled to a data line, such as those typically referred to as bit lines. In NAND flash architecture, a column of memory cells is coupled in series with only the first memory cell of the column coupled to a bit line.
As the performance and complexity of electronic systems increase, the requirement for additional memory in a system also increases. However, in order to continue to reduce the costs of the system, the parts count must be kept to a minimum. This can be accomplished by increasing the memory density of an integrated circuit by using such technologies as multilevel cells (MLC). For example, MLC NAND flash memory is a very cost effective non-volatile memory.
Multilevel cells can take advantage of the analog nature of a traditional flash cell by assigning a bit pattern to a specific threshold voltage (Vt) range stored on the cell. This technology permits the storage of two or more bits per cell, depending on the quantity of voltage ranges assigned to the cell and the stability of the assigned voltage ranges during the lifetime operation of the memory cell.
For example, a cell may be assigned four different voltage ranges of 200 mV for each range. Typically, a safety range of 0.2V to 0.4V is between each range to keep the ranges from overlapping. If the voltage stored on the cell is within the first range, the cell is in a first data state (representing, e.g., a logical 11), which is typically considered the erased state of the cell. If the voltage is within the second range, the cell is in a second data state (representing, e.g., a logical 01). This continues for as many ranges that are used for the cell provided these voltage ranges remain stable during the lifetime operation of the memory cell.
Since a MLC cell can be in one of two or more data states, the width of each of the voltage ranges for each state can be very important. The width is related to many variables in the operation of a memory circuit. In order to properly read a particular data state, a sense parameter such as a read voltage level, should be determined for the data state. A read voltage level, for example, can be affected by the width of an actual distribution of memory cells programmed to the corresponding data state within a memory, by threshold voltage noise, fluctuations around a transition point, which might also be referred to as a cross-over point, from one range to another, the width of threshold distributions (i.e., fat tails, that is distributions that extend into an adjacent distribution, e.g., the tail of the distribution is flared out, compared to a Gaussian distribution), and the like.
For reasons such as those stated above, and for other reasons, such as those stated below, which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for, among other things, improvements in determining a sense parameter for memories.